A constitutional example of a conventional LED drive unit, which drives an LED by a commercial AC source and is applicable to a lighting system, is described by referring to the drawings.
FIG. 8 is a schematic view showing one embodiment of an LED lighting device according to prior art. In the figure, there are provided a commercial AC source 400; a full-wave rectifier circuit 401 comprised of rectifier diodes 4011 to 4014 for rectifying the commercial AC source; a current-limiting resistor 402 connected to a plus output terminal A of the full-wave rectifier circuit 401 for limiting a current flowing through an LED array 403; the LED array 403 having N number (N≧1) of LEDs 4031 to 403N connected in series; a capacitor 404 for smoothing an output of the full-wave rectifier circuit (see FIG. 7 of Japanese Patent Laid-Open Publication No. H11-67,471, for example).
FIG. 9 shows another LED lighting device having a power transformer 506 provided therewith according to prior art. There are provided a current-limiting resistor 502 and a capacitor 504 for smoothing. In FIGS. 8 and 9, like reference numerals refer to like elements. The power transformer 506 is for stepping down a voltage of the commercial AC source to a required level, and this allows a modification in the number of LEDs 403 in series depending on the design purpose.
FIG. 10 shows still another LED lighting device according to prior art. For power conversion efficiency, the device has a DC/DC converter 604 with a transformer (not shown) built-in. There are provided a current-limiting resistor 602. Like numerals refer to like elements in FIGS. 8 and 10.
In the conventional constitution as shown in FIG. 8, the capacitor 404 with a high voltage and a large capacity has to be used to supply a voltage higher than the total sum of forward voltages of LEDs, disadvantageously resulting in a larger and expensive device. Further, the current-limiting resistor 402 incorporated in a current pathway to LEDs always has a current flow for lighting all the LEDs. Thus, a disadvantage with the device is that power consumption at the current-limiting resistor is large.
In the conventional constitution as shown in FIG. 9, the device has a large size and weight and is expensive due to the provision of the power transformer. Further, the device has a problem in that power consumption at the current-limiting resistor is still large.
In the conventional constitution as shown in FIG. 10, it is expected that power efficiency is improved due to the provision of the DC/DC converter in comparison with the constitutions of FIGS. 8 and 9. However, in addition to the DC/DC converter having a transformer built-in, an EMI filter for suppressing noises generated by the DC/DC converter is required, disadvantageously resulting in a larger and expensive device.
As described above, all of the LED lighting devices according to prior art cannot simultaneously solve both problems in terms of size/cost and power efficiency. Further, the device having a DC/DC converter additionally requires an anti-noise measure. The problem of power efficiency becomes particularly serious when a lighting system using white LEDs is designed as an alternative to an electric bulb or a fluorescent lamp.